An ink-jet recording method is a relatively simple apparatus and is capable of recording highly detailed images, leading to the rapid development in various fields. Further, an ink-jet recording method is used in a wide variety of applications, and a suitable recording medium or ink for each application is employed. Specifically, recording speed has been significantly increased, and then printers with performance capable of meeting quick printing applications have been developed in recent years.
However, in order for an ink-jet printer to exert its optimal performance, special ink-jet paper featuring appropriate ink absorptivity is critical.
When recording on coated paper or art paper with limited ink absorptivity or on a plastic film of no ink absorptivity, produced are problems such as bleeding, resulting in color mixing due to mixing of different liquid color inks on the recording medium, which has been a continuing problem in diversifying recording media in the ink-jet printing field.
To overcome the above problems, a hot-melt ink-jet recording method has been proposed, wherein a hot-melt ink composition containing wax, which is solid at room temperature, is used as a basic material; the composition is liquefied by heat, ejected via an appropriate form of energy, and deposited on a recording medium to form recorded dots which are cool-solidified during the deposition. Since this ink is solid at room temperature, no smudging during handling or clogging of the nozzle occurs since there is substantially no evaporation of the ink while in its melted state.
Further, an ink composition has been proposed which exhibits low color bleeding due to rapid solidification after deposition and provides excellent printing quality regardless of paper quality (for example, refer to Patent Documents 1 and 2).
However, since images recorded via these methods are composed of soft wax-like ink dots, problems of quality degradation due to dot embossment and of deficient abrasion resistance have been noted.
In contrast, an ink-jet UV ink has been disclosed, wherein an ink employing a colorant and a polymerizable material is ejected and cured via UV light irradiation (for example, refer to Patent Document 3). In this case, since all the ink is cured, a image portion produced is thick, resulting in a major difference in glossiness, compared to a non-image portion, which produces an unnatural image, whereby no natural image can be produced.
To eliminate glossiness difference, a solvent-based UV ink has been proposed, wherein a pigment and a polymerizable material are dissolved in an organic solvent (for example, refer to Patent Document 4). In this case, although the thickness of the image portion can be controlled and the problem of glossiness can also be improved, in contrast, there is an essential disadvantage in such that a poor working-environment, due to odor resulting from the organic solvent, is created. Therefore, the solvent-based UV ink has not been put into practice.
To improve the glossiness of the image portion and also to overcome the odor problem, there have been proposed water-based UV light curable inks employing a water-based UV curable monomer (for example, refer to Patent Document 5), and a water-soluble crosslinkable polymer (for example, refer to Patent Document 6). Although the glossiness of the image portion is improved and the odor problem is overcome using these inks, sufficient image quality is yet be realized in overcoming the specific problem of the UV ink.
The above specific problem of the UV ink is that, since a large amount of a colorant is contained in the ink, much of the UV light irradiated from a UV light source is absorbed by the colorant, resulting in requiring a large amount of light for curing. It has been widely demanded that, since the amount of light required for curing yellow ink, magenta ink, cyan ink, and black ink is commonly increased in the stated order, the photocuring sensitivity of the black ink is specifically enhanced. As a method of enhancing the photocuring sensitivity, it is known that sensitizers are added or longer-wavelength initiators are used as the photoinitiator. However, most of these substances are oil-based and no water-soluble substances have been found practicable.
Proposed substances include an initiator prepared by introducing a hydroxyethoxy group into 2-hydroxy-2-methyl-1-phenylpropane-1-one or benzophenone, as described in Unexamined Japanese Patent Application Publication No. (hereinafter, referred to as JP-A) 6-228218, an alkylphenone derivative whose α-position is substituted with a polyalkyleneoxide, as described in JP-A No. 2003-192712, a photopolymerization initiator which is a halomethyl-1,3,5-triazine anion, as described in JP-A No. 2001-525887, a water-soluble photoinitiator featuring a cycloalkanol structure, as described in JP-A No. 2000-273110, an ethylene oxide adduct of 2-hydroxy-2-methyl-1-phenylpropane-1-one, as described in JP-A No. 2000-186242, an acyl water-soluble-phosphine, as described in JP-A No. 2005-307199, and an ethylene oxide adduct of 2-hydroxy-2-methyl-1-phenylpropane-1-one, as described in U.S. Pat. No. 6,500,875.
However, there have been noted disadvantages in that only a small amount of these photoinitiators can be dissolved in water; or the number of moles (the number of molecules) of an initiator containable in the ink is reduced, since the introduction of a long polyethylene oxide group into an initiator for enhancing its solubility increases the molecular weight.
Further, a compound described in JP-A No. 2005-307199 has been proposed as a phosphine-based initiator, featuring an absorption sensitivity of a wavelength of up to 420 nm, which is considered to sufficiently cure even black ink. However, there are safety problems based on the Ames test, and since the initiator features an absorption sensitivity of a wavelength of up to 420 nm, then the feasibility of curing even under fluorescent lighting exists, whereby the initiator is not preferably used for the above water-based UV curable ink from the viewpoint of ink storage properties and working properties.    Patent Document 1: U.S. Pat. No. 4,390,369    Patent Document 2: U.S. Pat. No. 4,484,948    Patent Document 3: U.S. Pat. No. 4,228,438    Patent Document 4: Examined Japanese Patent Application Publication No. 5-64667    Patent Document 5: JP-A No. 7-224241    Patent Document 6: PCT International Application Publication of WO 06/80139